This invention relates to a method of manufacturing a press lens which, after completion of the manufacturing steps, is so high both in configuration accuracy and in surface roughness that it is unnecessary to polish the lens.
Glass lenses are manufactured according to previously known methods in which glass is melted or softened, put into a metal mold, and the glass in the mold is pressed to have an external form which is generally similar to or approximating the desired final external form of the lens desired. The pressed lens is then subjected to cold grinding and polishing to make a finished lens.
The art has also described a procedure for pressing a lens having optical mirror surfaces which needs no grinding and polishing steps during the manufacturing in which a mirror-finished mold, is used to form the lens by pressing the lens-forming material in a non-oxidizing atmosphere. Glasslike carbon is disclosed as a suitable material for fabricating the mold in the specification of Japanese Patent Application Laid-Open No. 11277/1972, SiC or Si.sub.3 N.sub.4 are suggested as suitable mold materials in the specification of Japanese Patent Application Laid-Open No. 45613/1977, and a mixture of SiC and carbon are described as suitable mold materials in the specification of U.S. Pat. No. 4,168,961. It will be appreciated that if a lens can be manufactured in final form according to such a method, then the manufacturing cost can be greatly reduced and the processing time simplified because no grinding or polishing steps are required in the manufacturing process.
The following two methods of imparting an external form to a piece of glass which is substantially similar to the desired final external form are well known in the art. In the first method, molten glass having a viscosity of 10 to 10.sup.3 poises is caused to drop in the form of a gob from the outlet, and the glass thus dropped, which is received by a mold at a temperature lower than the glass transition temperature, is pressed with a pressure of 2 to 10 kg/cm.sup.2. In this operation, the mold serves to shape the glass and to receive heat from the high temperature glass during pressing. The temperature of the mold is controlled and made lower than the glass transition temperature to prevent the mold from fusing with the glass. In this case, the pressing operation is stopped before the inside of the piece of glass thus formed is sufficiently solidified although the surface of the glass piece is cooled and solidified. Because of the difference in contraction between the cooled surface and the warmer inside of the glass piece, a so-called "shrink phenomenon" occurs with the piece of glass, and therefore the piece of glass thus formed is low in configuration accuracy. Indeed, if the pressing operation is carried out for a longer period of time at a low mold temperature, then the surface of the piece of glass is cooler than the inside thereof, and the piece of glass is easily cracked.
In the second method, a piece of glass stock such as a glass plate, a glass block or a glass bar, is cut to obtain a piece of glass having a predetermined volume dimensions, and the piece of glass thus obtained is coated with a release agent such as Al.sub.2 O.sub.3 or BN. The piece of glass thus treated is put in an oven where it is softened to about 10.sup.5 poises and, thus softened, the piece of glass is quickly put in a mold and is pressed by a pressure of 5 to 50 kg/cm.sup.2. In this method, the related agent sticks to the surface of the product, and therefore it is necessary to grind and polish the product. On the other hand, in the press lens forming method disclosed by Japanese Patent Application Laid-Open No. 11277/1972 and U.S. Pat. No. 4,046,545, mentioned above, a piece of glass is put in a mold and it is pressed while it is being heated together with the mold. While the piece of glass together with and retained in the mold is being cooled, the pressing operation is continued until the temperature of the piece of glass becomes lower than the glass transition temperature. However, in this method, the cycle time to make a single pressed lens is considerably long, and the glass must be kept in contact with the mold for the long cycle time, as a result of which the surface of the mold is liable to be roughened unless special glass or a special mold material is used.